effective-go

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 Table of Contents 前言 引言 格式化 注释 命名 分号 控制结构 函数 数据 初始化 方法 接口和其他类型 空白标识符 内嵌 并发 错误 一个Web服务器 2 《Effective Go》中英双语版 Effective Go - 《实效 GO 编程》 Introduction Go is a new language. Although it borrows ideas from existing languages, it has unusual properties that make effective Go programs different in character from programs written in its relatives. A straightforward translation of a C++ or Java program into Go is unlikely to produce a satisfactory result—Java programs are written in Java, not Go. On the other hand, thinking about the problem from a Go perspective could produce a successful but quite different program. In other words, to write Go well, it's important to understand its properties and idioms. It's also important to know the established conventions for programming in Go, such as naming, formatting, program construction, and so on, so that programs you write will be easy for other Go programmers to understand. This document gives tips for writing clear, idiomatic Go code. It augments the language specification, the Tour of Go, and How to Write Go Code, all of which you should read first. 引言 Go 是一门全新的语言。尽管它从既有的语言中借鉴了许多理念，但其与众不同的特性，使得 用 Go 编程在本质上就不同于其它语言。将现有的 C++ 或 Java 程序直译为 Go 程序并不能令 人满意——毕竟 Java 程序是用 Java 编写的，而不是 Go。 另一方面，若从 Go 的角度去分 析问题，你就能编写出同样可行但大不相同的程序。 换句话说，要想将 Go 程序写得好，就 必须理解其特性和风格。了解命名、格式化、程序结构等既定规则也同样重要，这样你编写 的程序才能更容易被其他程序员所理解。 本文档就如何编写清晰、地道的 Go 代码提供了一些技巧。它是对 语言规范、 Go 语言之旅 以及 如何使用 Go 编程 的补充说明，因此我们建议您先阅读这些文档。 Effective Go 作为 GO 语言的入门必读教程，值得每位初学者好好阅读一遍，编辑成书，方 便阅读交流。 当前完成章节（全部完成）： 前言 3 1. 前言 2. 引言 3. 格式化 4. 注释 5. 命名 6. 分号 7. 控制结构 8. 函数 9. 数据 10. 初始化 11. 方法 12. 接口和其他类型 13. 空白标识符 14. 内嵌 15. 并发 16. 错误 17. 一个 Web 服务器 改版说明：@2016.8.6 by bingoHuang, revision to Chinese & English version. 李笑来在他的新书 《人人都是工程师》 中说过一句话： 在中国，对绝大多数人来 说， English + Computer Skills = Freedom （英语 + 计算机技能 = 自由） 我非常的赞同。英语和计算机技能是相辅相成，学习好一门编程语言（如 Go ）的同 时，还能加强英语学习，何乐而不为。所以我决定将本书改版成中英双语版，方便更多 的人来学习阅读。 特别感谢 Golang 官网提供的英文版教程。 感谢 hellogcc 提供的 中文翻译版一，这是我之前制作中文版电子书所参考的资料，翻译 的很用心。 要更感谢 Go 语言中文社区 提供的 中文翻译版二，此翻译更贴切有味道，不可多得。本 人已和 Go-zh 项目组 沟通过，获取了该社区的授权，故将此作为双语版的中文版本。 参考 参考官方英文版：Effective Go 英文版 参考中文翻译版一：Effective Go 中文版 参考中文翻译版二：Effective Go 中文版 Read, Fork and Star 前言 4 Read on Gitbook Fork on GitHub 请顺手点一下 STAR ，或者留言讨论，这是对我最大的鼓励！ 下载 为了让大家更方便阅读，在此提供 网易蜂巢对象存储 的下载地址： [x] PDF格式 [x] EPUB格式 License 除特别注明外， 本页内容均采用知识共享 - 署名（CC-BY）3.0 协议授权，代码采用 BSD 协 议 授权。 前言 5 Introduction 引言 Go is a new language. Although it borrows ideas from existing languages, it has unusual properties that make effective Go programs different in character from programs written in its relatives. A straightforward translation of a C++ or Java program into Go is unlikely to produce a satisfactory result—Java programs are written in Java, not Go. On the other hand, thinking about the problem from a Go perspective could produce a successful but quite different program. In other words, to write Go well, it's important to understand its properties and idioms. It's also important to know the established conventions for programming in Go, such as naming, formatting, program construction, and so on, so that programs you write will be easy for other Go programmers to understand. Go 是一门全新的语言。尽管它从既有的语言中借鉴了许多理念，但其与众不同的特性， 使得 使用 Go 编程在本质上就不同于其它语言。将现有的 C++ 或 Java 程序直译为 Go 程序并不能 令人满意——毕竟 Java 程序是用 Java 编写的，而不是 Go。 另一方面，若从 Go 的角度去 分析问题，你就能编写出同样可行但大不相同的程序。 换句话说，要想将 Go 程序写得好， 就必须理解其特性和风格。了解命名、格式化、 程序结构等既定规则也同样重要，这样你编 写的程序才能更容易被其他程序员所理解。 This document gives tips for writing clear, idiomatic Go code. It augments the language specification, the Tour of Go, and How to Write Go Code, all of which you should read first. 本文档就如何编写清晰、地道的 Go 代码提供了一些技巧。它是对 语言规范、 Go 语言之旅 以及 如何使用 Go 编程 的补充说明，因此我们建议您先阅读这些文档。 Examples 示例 The Go package sources are intended to serve not only as the core library but also as examples of how to use the language. Moreover, many of the packages contain working, self-contained executable examples you can run directly from the golang.org web site, such as this one (if necessary, click on the word"Example"to open it up). If you have a question about how to approach a problem or how something might be implemented, the documentation, code and examples in the library can provide answers, ideas and background. 引言 6 Go 包的源码 不仅是核心库，同时也是学习如何使用 Go 语言的示例源码。 此外，其中的一 些包还包含了可工作的，独立的可执行示例，你可以直接在 golang.org 网站上运行它们，比 如 这个例子 （单击文字 “示例” 来展开它）。如果你有任何关于某些问题如何解决，或某些东 西如何实现的疑问， 也可以从中获取相关的答案、思路以及后台实现。 引言 7 Formatting 格式化 Formatting issues are the most contentious but the least consequential. People can adapt to different formatting styles but it's better if they don't have to, and less time is devoted to the topic if everyone adheres to the same style. The problem is how to approach this Utopia without a long prescriptive style guide. 格式化问题总是充满了争议，但却始终没有形成统一的定论。虽说人们可以适应不同的编码 风格， 但抛弃这种适应过程岂不更好？若所有人都遵循相同的编码风格，在这类问题上浪费 的时间将会更少。 问题就在于如何实现这种设想，而无需冗长的语言风格规范。 With Go we take an unusual approach and let the machine take care of most formatting issues. The gofmt program (also available as go fmt, which operates at the package level rather than source file level) reads a Go program and emits the source in a standard style of indentation and vertical alignment, retaining and if necessary reformatting comments. If you want to know how to handle some new layout situation, run gofmt; if the answer doesn't seem right, rearrange your program (or file a bug about gofmt), don't work around it. 在 Go 中我们另辟蹊径，让机器来处理大部分的格式化问题。gofmt 程序（也可用 go fmt，它 以包为处理对象而非源文件）将 Go 程序按照标准风格缩进、 对齐，保留注释并在需要时重 新格式化。若你想知道如何处理一些新的代码布局，请尝试运行 gofmt；若结果仍不尽人意， 请重新组织你的程序（或提交有关 gofmt 的 Bug），而不必为此纠结。 As an example, there's no need to spend time lining up the comments on the fields of a structure. Gofmt will do that for you. Given the declaration 举例来说，你无需花时间将结构体中的字段注释对齐，gofmt 将为你代劳。 假如有以下声 明： type T struct { name string // name of the object value int // its value } type T struct { name string // 对象名 value int // 对象值 } 格式化 8 gofmt will line up the columns: gofmt 会将它按列对齐为： type T struct { name string // name of the object value int // its value } type T struct { name string // 对象名 value int // 对象值 } All Go code in the standard packages has been formatted with gofmt. 标准包中所有的 Go 代码都已经用 gofmt 格式化过了。 Some formatting details remain. Very briefly: 还有一些关于格式化的细节，它们非常简短： Indentation We use tabs for indentation and gofmt emits them by default. Use spaces only if you must. Line length Go has no line length limit. Don't worry about overflowing a punched card. If a line feels too long, wrap it and indent with an extra tab. Parentheses Go needs fewer parentheses than C and Java: control structures (if, for, switch) do not have parentheses in their syntax. Also, the operator precedence hierarchy is short er and clearer, so x<<8 + y<<16 means what the spacing implies, unlike in the other languages. 缩进 我们使用制表符（tab）缩进，gofmt 默认也使用它。在你认为确实有必要时再使用空格。 行的长度 Go 对行的长度没有限制，别担心打孔纸不够长。如果一行实在太长，也可进行折行并插入适当的 tab 缩进。 括号 比起 C 和 Java，Go 所需的括号更少：控制结构（if、for 和 switch）在语法上并不需要圆括号。此外，操 作符优先级处理变得更加简洁，因此 x<<8 + y<<16 正表述了空格符所传达的含义。 格式化 9 格式化 10 Commentary 注释 Go provides C-style /* */ block comments and C++-style // line comments. Line comments are the norm; block comments appear mostly as package comments, but are useful within an expression or to disable large swaths of code. Go 语言支持 C 风格的块注释 /* */ 和 C++ 风格的行注释 // 。 行注释更为常用，而块注 释则主要用作包的注释，当然也可在禁用一大段代码时使用。 The program—and web server—godoc processes Go source files to extract documentation about the contents of the package. Comments that appear before top-level declarations, with no intervening newlines, are extracted along with the declaration to serve as explanatory text for the item. The nature and style of these comments determines the quality of the documentation godoc produces. godoc 既是一个程序，又是一个 Web 服务器，它对 Go 的源码进行处理，并提取包中的文档 内容。 出现在顶级声明之前，且与该声明之间没有空行的注释，将与该声明一起被提取出 来，作为该条目的说明文档。 这些注释的类型和风格决定了 godoc 生成的文档质量。 Every package should have a package comment, a block comment preceding the package clause. For multi-file packages, the package comment only needs to be present in one file, and any one will do. The package comment should introduce the package and provide information relevant to the package as a whole. It will appear first on the godoc page and should set up the detailed documentation that follows. 每个包都应包含一段包注释，即放置在包子句前的一个块注释。对于包含多个文件的包， 包 注释只需出现在其中的任一文件中即可。包注释应在整体上对该包进行介绍，并提供包的相 关信息。 它将出现在 godoc 页面中的最上面，并为紧随其后的内容建立详细的文档。 注释 11 /* Package regexp implements a simple library for regular expressions. The syntax of the regular expressions accepted is: regexp: concatenation { '|' concatenation } concatenation: { closure } closure: term [ '*' | '+' | '?' ] term: '^' '$' '.' character '[' [ '^' ] character-ranges ']' '(' regexp ')' */ package regexp /* regexp 包为正则表达式实现了一个简单的库。 该库接受的正则表达式语法为： 正则表达式: 串联 { '|' 串联 } 串联: { 闭包 } 闭包: 条目 [ '*' | '+' | '?' ] 条目: '^' '$' '.' 字符 '[' [ '^' ] 字符遍历 ']' '(' 正则表达式 ')' */ package regexp If the package is simple, the package comment can be brief. 若某个包比较简单，包注释同样可以简洁些。 // Package path implements utility routines for // manipulating slash-separated filename paths. 注释 12 // path 包实现了一些常用的工具，以便于操作用反斜杠分隔的路径. Comments do not need extra formatting such as banners of stars. The generated output may not even be presented in a fixed-width font, so don't depend on spacing for alignment— godoc, like gofmt, takes care of that. The comments are uninterpreted plain text, so HTML and other annotations such as _this_ will reproduce verbatim and should not be used. One adjustment godoc does do is to display indented text in a fixed-width font, suitable for program snippets. The package comment for the fmt package uses this to good effect. 注释无需进行额外的格式化，如用星号来突出等。生成的输出甚至可能无法以等宽字体显 示， 因此不要依赖于空格对齐，godoc 会像 gofmt 那样处理好这一切。 注释是不会被解析的 纯文本，因此像 HTML 或其它类似于 _这样_ 的东西将按照 原样 输出，因此不应使用它们。 godoc 所做的调整， 就是将已缩进的文本以等宽字体显示，来适应对应的程序片段。 fmt 包 的注释就用了这种不错的效果。 Depending on the context, godoc might not even reformat comments, so make sure they look good straight up: use correct spelling, punctuation, and sentence structure, fold long lines, and so on. godoc 是否会重新格式化注释取决于上下文，因此必须确保它们看起来清晰易辨： 使用正确 的拼写、标点和语句结构以及折叠长行等。 Inside a package, any comment immediately preceding a top-level declaration serves as a doc comment for that declaration. Every exported (capitalized) name in a program should have a doc comment. 在包中，任何顶级声明前面的注释都将作为该声明的文档注释。 在程序中，每个可导出（首 字母大写）的名称都应该有文档注释。 Doc comments work best as complete sentences, which allow a wide variety of automated presentations. The first sentence should be a one-sentence summary that starts with the name being declared. 文档注释最好是完整的句子，这样它才能适应各种自动化的展示。 第一句应当以被声明的东 西开头，并且是单句的摘要。 // Compile parses a regular expression and returns, if successful, a Regexp // object that can be used to match against text. func Compile(str string) (regexp *Regexp, err error) { // Compile 用于解析正则表达式并返回，如果成功，则 Regexp 对象就可用于匹配所针对的文本。 func Compile(str string) (regexp *Regexp, err error) { 注释 13 If the name always begins the comment, the output of godoc can usefully be run through grep. Imagine you couldn't remember the name"Compile" but were looking for the parsing function for regular expressions, so you ran the command, 若注释总是以名称开头，godoc 的输出就能通过 grep 变得更加有用。假如你记不住 “Compile” 这个名称，而又在找正则表达式的解析函数， 那就可以运行 $ godoc regexp | grep parse If all the doc comments in the package began, "This function...", grep wouldn't help you remember the name. But because the package starts each doc comment with the name, you'd see something like this, which recalls the word you're looking for. 若包中的所有文档注释都以 “此函数…” 开头，grep 就无法帮你记住此名称。 但由于每个包的 文档注释都以其名称开头，你就能看到这样的内容，它能显示你正在寻找的词语。 $ godoc regexp | grep parse Compile parses a regular expression and returns, if successful, a Regexp parsed. It simplifies safe initialization of global variables holding cannot be parsed. It simplifies safe initialization of global variables $ Go's declaration syntax allows grouping of declarations. A single doc comment can introduce a group of related constants or variables. Since the whole declaration is presented, such a comment can often be perfunctory. Go的声明语法允许成组声明。单个文档注释应介绍一组相关的常量或变量。 由于是整体声 明，这种注释往往较为笼统。 // Error codes returned by failures to parse an expression. var ( ErrInternal = errors.New("regexp: internal error") ErrUnmatchedLpar = errors.New("regexp: unmatched '('") ErrUnmatchedRpar = errors.New("regexp: unmatched ')'") ... ) // 表达式解析失败后返回错误代码。 var ( ErrInternal = errors.New("regexp: internal error") ErrUnmatchedLpar = errors.New("regexp: unmatched '('") ErrUnmatchedRpar = errors.New("regexp: unmatched ')'") ... ) 注释 14 Grouping can also indicate relationships between items, such as the fact that a set of variables is protected by a mutex. 即便是对于私有名称，也可通过成组声明来表明各项间的关系，例如某一组由互斥体保护的 变量。 var ( countLock sync.Mutex inputCount uint32 outputCount uint32 errorCount uint32 ) 注释 15 Names 命名 Names are as important in Go as in any other language. They even have semantic effect: the visibility of a name outside a package is determined by whether its first character is upper case. It's therefore worth spending a little time talking about naming conventions in Go programs. 正如命名在其它语言中的地位，它在 Go 中同样重要。有时它们甚至会影响语义： 例如，某 个名称在包外是否可见，就取决于其首个字符是否为大写字母。 因此有必要花点时间来讨论 Go 程序中的命名约定。 Package names 包名 When a package is imported, the package name becomes an accessor for the contents. After 当一个包被导入后，包名就会成了内容的访问器。在 import "bytes" the importing package can talk about bytes.Buffer. It's helpful if everyone using the package can use the same name to refer to its contents, which implies that the package name should be good: short, concise, evocative. By convention, packages are given lower case, single- word names; there should be no need for underscores or mixedCaps. Err on the side of brevity, since everyone using your package will be typing that name. And don't worry about collisions a priori. The package name is only the default name for imports; it need not be unique across all source code, and in the rare case of a collision the importing package can choose a different name to use locally. In any case, confusion is rare because the file name in the import determines just which package is being used. 之后，被导入的包就能通过 bytes.Buffer 来引用了。 若所有人都以相同的名称来引用其内容 将大有裨益， 这也就意味着包应当有个恰当的名称：其名称应该简洁明了而易于理解。按照 惯例， 包应当以小写的单个单词来命名，且不应使用下划线或驼峰记法。err 的命名就是出于 简短考虑的，因为任何使用该包的人都会键入该名称。 不必担心引用次序的冲突。包名就是 命名 16 导入时所需的唯一默认名称， 它并不需要在所有源码中保持唯一，即便在少数发生冲突的情 况下， 也可为导入的包选择一个别名来局部使用。 无论如何，通过文件名来判定使用的包， 都是不会产生混淆的。 Another convention is that the package name is the base name of its source directory; the package in src/encoding/base64 is imported as "encoding/base64" but has name base64, not encoding_base64 and not encodingBase64. 另一个约定就是包名应为其源码目录的基本名称。在 src/pkg/encoding/base64 中的包应作为 "encoding/base64" 导入，其包名应为 base64， 而非 encoding_base64 或 encodingBase64。 The importer of a package will use the name to refer to its contents, so exported names in the package can use that fact to avoid stutter. (Don't use the import . notation, which can simplify tests that must run outside the package they are testing, but should otherwise be avoided.) For instance, the buffered reader type in the bufio package is called Reader, not BufReader, because users see it as bufio.Reader, which is a clear, concise name. Moreover, because imported entities are always addressed with their package name, bufio.Reader does not conflict with io.Reader. Similarly, the function to make new instances of ring.Ring— which is the definition of a constructor in Go—would normally be called NewRing, but since Ring is the only type exported by the package, and since the package is called ring, it's called just New, which clients of the package see as ring.New. Use the package structure to help you choose good names. 包的导入者可通过包名来引用其内容，因此包中的可导出名称可以此来避免冲突。 （请勿使 用 import . 记法，它可以简化必须在被测试包外运行的测试， 除此之外应尽量避免使用。） 例如，bufio 包中的缓存读取器类型叫做 Reader 而非 BufReader，因为用户将它看做 bufio.Reader，这是个清楚而简洁的名称。 此外，由于被导入的项总是通过它们的包名来确 定，因此 bufio.Reader 不会与 io.Reader 发生冲突。同样，用于创建 ring.Ring 的新实例的函 数（这就是 Go 中的构造函数）一般会称之为 NewRing，但由于 Ring 是该包所导出的唯一类 型，且该包也叫 ring，因此它可以只叫做 New，它跟在包的后面，就像 ring.New。使用包结 构可以帮助你选择好的名称。 Another short example is once.Do; once.Do(setup) reads well and would not be improved by writing once.DoOrWaitUntilDone(setup). Long names don't automatically make things more readable. A helpful doc comment can often be more valuable than an extra long name. 另一个简短的例子是 once.Do，once.Do(setup) 表述足够清晰， 使用 once.DoOrWaitUntilDone(setup) 完全就是画蛇添足。 长命名并不会使其更具可读性。一份有 用的说明文档通常比额外的长名更有价值。 Getters 命名 17 获取器 Go doesn't provide automatic support for getters and setters. There's nothing wrong with providing getters and setters yourself, and it's often appropriate to do so, but it's neither idiomatic nor necessary to put Get into the getter's name. If you have a field called owner (lower case, unexported), the getter method should be called Owner (upper case, exported), not GetOwner. The use of upper-case names for export provides the hook to discriminate the field from the method. A setter function, if needed, will likely be called SetOwner. Both names read well in practice: Go 并不对获取器（getter）和设置器（setter）提供自动支持。 你应当自己提供获取器和设 置器，通常很值得这样做，但若要将 Get 放到获取器的名字中，既不符合习惯，也没有必 要。若你有个名为 owner （小写，未导出）的字段，其获取器应当名为 Owner（大写，可导 出）而非 GetOwner。大写字母即为可导出的这种规定为区分方法和字段提供了便利。 若要 提供设置器方法，SetOwner 是个不错的选择。两个命名看起来都很合理： owner := obj.Owner() if owner != user { obj.SetOwner(user) } Interface names 接口名 By convention, one-method interfaces are named by the method name plus an -er suffix or similar modification to construct an agent noun: Reader, Writer, Formatter, CloseNotifier etc. 按照约定，只包含一个方法的接口应当以该方法的名称加上 - er 后缀来命名，如 Reader、 Writer、 Formatter、CloseNotifier 等。 There are a number of such names and it's productive to honor them and the function names they capture. Read, Write, Close, Flush, String and so on have canonical signatures and meanings. To avoid confusion, don't give your method one of those names unless it has the same signature and meaning. Conversely, if your type implements a method with the same meaning as a method on a well-known type, give it the same name and signature; call your string-converter method String not ToString. 诸如此类的命名有很多，遵循它们及其代表的函数名会让事情变得简单。 Read、Write、 Close、Flush、 String 等都具有典型的签名和意义。为避免冲突，请不要用这些名称为你的 方法命名， 除非你明确知道它们的签名和意义相同。反之，若你的类型实现了的方法， 与一 个众所周知的类型的方法拥有相同的含义，那就使用相同的命名。 请将字符串转换方法命名 为 String 而非 ToString。 命名 18 MixedCaps 驼峰记法 Finally, the convention in Go is to use MixedCaps or mixedCaps rather than underscores to write multiword names. 最后，Go 中约定使用驼峰记法 MixedCaps 或 mixedCaps。 命名 19 Semicolons 分号 Like C, Go's formal grammar uses semicolons to terminate statements, but unlike in C, those semicolons do not appear in the source. Instead the lexer uses a simple rule to insert semicolons automatically as it scans, so the input text is mostly free of them. 和 C 一样，Go 的正式语法使用分号来结束语句；和 C 不同的是，这些分号并不在源码中出 现。 取而代之，词法分析器会使用一条简单的规则来自动插入分号，因此源码中基本就不用 分号了。 The rule is this. If the last token before a newline is an identifier (which includes words like int and float64), a basic literal such as a number or string constant, or one of the tokens 规则是这样的：若在新行前的最后一个标记为标识符（包括 int 和 float64 这类的单词）、数 值或字符串常量之类的基本字面或以下标记之一 break continue fallthrough return ++ -- ) } the lexer always inserts a semicolon after the token. This could be summarized as, “if the newline comes after a token that could end a statement, insert a semicolon”. 则词法分析将始终在该标记后面插入分号。这点可以概括为： “如果新行前的标记为语句的末 尾，则插入分号”。 A semicolon can also be omitted immediately before a closing brace, so a statement such as 分号也可在闭括号之前直接省略，因此像 go func() { for { dst 0 { return y } Mandatory braces encourage writing simple if statements on multiple lines. It's good style to do so anyway, especially when the body contains a control statement such as a return or break. 强制的大括号促使你将简单的 if 语句分成多行。特别是在主体中包含 return 或 break 等控制 语句时，这种编码风格的好处一比便知。 Since if and switch accept an initialization statement, it's common to see one used to set up a local variable. 由于 if 和 switch 可接受初始化语句， 因此用它们来设置局部变量十分常见。 控制结构 22 if err := file.Chmod(0664); err != nil { log.Print(err) return err } In the Go libraries, you'll find that when an if statement doesn't flow into the next statement— that is, the body ends in break, continue, goto, or return—the unnecessary else is omitted. 在 Go 的库中，你会发现若 if 语句不会执行到下一条语句时，亦即其执行体 以 break、 continue、goto 或 return 结束时，不必要的 else 会被省略。 f, err := os.Open(name) if err != nil { return err } codeUsing(f) This is an example of a common situation where code must guard against a sequence of error conditions. The code reads well if the successful flow of control runs down the page, eliminating error cases as they arise. Since error cases tend to end in return statements, the resulting code needs no else statements. 下例是一种常见的情况，代码必须防范一系列的错误条件。若控制流成功继续， 则说明程序 已排除错误。由于出错时将以 return 结束， 之后的代码也就无需 else 了。 f, err := os.Open(name) if err != nil { return err } d, err := f.Stat() if err != nil { f.Close() return err } codeUsing(f, d) Redeclaration and reassignment 重新声明与再次赋值 An aside: The last example in the previous section demonstrates a detail of how the := short declaration form works. The declaration that calls os.Open reads, 题外话：上一节中最后一个示例展示了短声明 := 如何使用。 调用了 os.Open 的声明为 控制结构 23 f, err := os.Open(name) This statement declares two variables, f and err. A few lines later, the call to f.Stat reads, 该语句声明了两个变量 f 和 err。在几行之后，又通过 d, err := f.Stat() which looks as if it declares d and err. Notice, though, that err appears in both statements. This duplication is legal: err is declared by the first statement, but only re-assigned in the second. This means that the call to f.Stat uses the existing err variable declared above, and just gives it a new value. 调用了 f.Stat。它看起来似乎是声明了 d 和 err。 注意，尽管两个语句中都出现了 err，但这 种重复仍然是合法的：err 在第一条语句中被声明，但在第二条语句中只是被再次赋值罢了。 也就是说，调用 f.Stat 使用的是前面已经声明的 err，它只是被重新赋值了而已。 In a := declaration a variable v may appear even if it has already been declared, provided: 在满足下列条件时，已被声明的变量 v 可出现在:= 声明中： this declaration is in the same scope as the existing declaration of v (if v is already declared in an outer scope, the declaration will create a new variable §), the corresponding value in the initialization is assignable to v, and there is at least one other variable in the declaration that is being declared anew. 本次声明与已声明的 v 处于同一作用域中（若 v 已在外层作用域中声明过，则此次声明 会创建一个新的变量 §）， 在初始化中与其类型相应的值才能赋予 v，且 在此次声明中至少另有一个变量是新声明的。 This unusual property is pure pragmatism, making it easy to use a single err value, for example, in a long if-else chain. You'll see it used often. 这个特性简直就是纯粹的实用主义体现，它使得我们可以很方面地只使用一个 err 值，例如， 在一个相当长的 if-else 语句链中， 你会发现它用得很频繁。 § It's worth noting here that in Go the scope of function parameters and return values is the same as the function body, even though they appear lexically outside the braces that enclose the body. § 值得一提的是，即便 Go 中的函数形参和返回值在词法上处于大括号之外， 但它们的作用 域和该函数体仍然相同。 控制结构 24 For The Go for loop is similar to—but not the same as—C's. It unifies for and while and there is no do-while. There are three forms, only one of which has semicolons. Go 的 for 循环类似于 C，但却不尽相同。它统一了 for 和 while，不再有 do-while 了。它有三 种形式，但只有一种需要分号。 // Like a C for for init; condition; post { } // Like a C while for condition { } // Like a C for(;;) for { } // 如同 C 的 for 循环 for init; condition; post { } // 如同 C 的 while 循环 for condition { } // 如同 C 的 for(;;) 循环 for { } Short declarations make it easy to declare the index variable right in the loop. 简短声明能让我们更容易在循环中声明下标变量： sum := 0 for i := 0; i < 10; i++ { sum += i } If you're looping over an array, slice, string, or map, or reading from a channel, a range clause can manage the loop. 若你想遍历数组、切片、字符串或者映射，或从信道中读取消息， range 子句能够帮你轻松 实现循环。 for key, value := range oldMap { newMap[key] = value } 控制结构 25 If you only need the first item in the range (the key or index), drop the second: 若你只需要该遍历中的第一个项（键或下标），去掉第二个就行了： for key := range m { if key.expired() { delete(m, key) } } If you only need the second item in the range (the value), use the blank identifier, an underscore, to discard the first: 若你只需要该遍历中的第二个项（值），请使用空白标识符，即下划线来丢弃第一个值： sum := 0 for _, value := range array { sum += value } The blank identifier has many uses, as described in a later section. 空白标识符还有多种用法，它会在后面的小节中描述。 For strings, the range does more work for you, breaking out individual Unicode code points by parsing the UTF-8. Erroneous encodings consume one byte and produce the replacement rune U+FFFD. (The name (with associated builtin type) rune is Go terminology for a single Unicode code point. See the language specification for details.) The loop 对于字符串，range 能够提供更多便利。它能通过解析 UTF-8， 将每个独立的 Unicode 码点 分离出来。错误的编码将占用一个字节，并以符文 U+FFFD 来代替。 （名称 “符文” 和内建类 型 rune 是 Go 对单个 Unicode 码点的成称谓。 详情见语言规范）。循环 for pos, char := range "日本 \ x80 語" { // \x80 is an illegal UTF-8 encoding fmt.Printf("character %#U starts at byte position %d\n", char, pos) } prints character U+65E5 '日' starts at byte position 0 character U+672C '本' starts at byte position 3 character U+FFFD '?' starts at byte position 6 character U+8A9E '語' starts at byte position 7 控制结构 26 for pos, char := range "日本\x80語" { // \x80 是个非法的UTF-8编码 fmt.Printf("字符 %#U 始于字节位置 %d\n", char, pos) } 将打印 字符 U+65E5 '日' 始于字节位置 0 字符 U+672C '本' 始于字节位置 3 字符 U+FFFD '?' 始于字节位置 6 字符 U+8A9E '語' 始于字节位置 7 Finally, Go has no comma operator and ++ and -- are statements not expressions. Thus if you want to run multiple variables in a for you should use parallel assignment (although that precludes ++ and --). 最后，Go 没有逗号操作符，而 ++ 和 -- 为语句而非表达式。 因此，若你想要在 for 中使用多 个变量，应采用平行赋值的方式 （因为它会拒绝 ++ 和 --）. // Reverse a for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 { a[i], a[j] = a[j], a[i] } // 反转 a for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 { a[i], a[j] = a[j], a[i] } Switch Go's switch is more general than C's. The expressions need not be constants or even integers, the cases are evaluated top to bottom until a match is found, and if the switch has no expression it switches on true. It's therefore possible—and idiomatic—to write an if-else- if-else chain as a switch. Go 的 switch 比 C 的更通用。其表达式无需为常量或整数，case 语句会自上而下逐一进行求 值直到匹配为止。若 switch 后面没有表达式，它将匹配 true，因此，我们可以将 if-else-if- else 链写成一个 switch，这也更符合 Go 的风格。 控制结构 27 func unhex(c byte) byte { switch { case '0' <= c && c <= '9': return c - '0' case 'a' <= c && c <= 'f': return c - 'a' + 10 case 'A' <= c && c <= 'F': return c - 'A' + 10 } return 0 } There is no automatic fall through, but cases can be presented in comma-separated lists. switch 并不会自动下溯，但 case 可通过逗号分隔来列举相同的处理条件。 func shouldEscape(c byte) bool { switch c { case ' ', '?', '&', '=', '#', '+', '%': return true } return false } Although they are not nearly as common in Go as some other C-like languages, break statements can be used to terminate a switch early. Sometimes, though, it's necessary to break out of a surrounding loop, not the switch, and in Go that can be accomplished by putting a label on the loop and"breaking" to that label. This example shows both uses. 尽管它们在 Go 中的用法和其它类 C 语言差不多，但 break 语句可以使 switch 提前终止。不 仅是 switch， 有时候也必须打破层层的循环。在 Go 中，我们只需将标签放置到循环外，然 后 “蹦” 到那里即可。下面的例子展示了二者的用法。 控制结构 28 Loop: for n := 0; n < len(src); n += size { switch { case src[n] < sizeOne: if validateOnly { break } size = 1 update(src[n]) case src[n] < sizeTwo: if n+1 >= len(src) { err = errShortInput break Loop } if validateOnly { break } size = 2 update(src[n] + src[n+1]< b func Compare(a, b []byte) int { for i := 0; i < len(a) && i < len(b); i++ { switch { case a[i] > b[i]: return 1 case a[i] < b[i]: return -1 } } switch { case len(a) > len(b): return 1 case len(a) < len(b): return -1 } return 0 } 控制结构 29 作为这一节的结束，此程序通过使用两个 switch 语句对字节数组进行比较： // Compare 按字典顺序比较两个字节切片并返回一个整数。 // 若 a == b，则结果为零；若 a < b；则结果为 -1；若 a > b，则结果为 +1。 func Compare(a, b []byte) int { for i := 0; i < len(a) && i < len(b); i++ { switch { case a[i] > b[i]: return 1 case a[i] < b[i]: return -1 } } switch { case len(a) > len(b): return 1 case len(a) < len(b): return -1 } return 0 } Type switch 类型选择 A switch can also be used to discover the dynamic type of an interface variable. Such a type switch uses the syntax of a type assertion with the keyword type inside the parentheses. If the switch declares a variable in the expression, the variable will have the corresponding type in each clause. It's also idiomatic to reuse the name in such cases, in effect declaring a new variable with the same name but a different type in each case. var t interface{} t = functionOfSomeType() switch t := t.(type) { default: fmt.Printf("unexpected type %T", t) // %T prints whatever type t has case bool: fmt.Printf("boolean %t\n", t) // t has type bool case int: fmt.Printf("integer %d\n", t) // t has type int case *bool: fmt.Printf("pointer to boolean %t\n", *t) // t has type *bool case *int: fmt.Printf("pointer to integer %d\n", *t) // t has type *int } 控制结构 30 switch 也可用于判断接口变量的动态类型。如 类型选择 通过圆括号中的关键字 type 使用类 型断言语法。若 switch 在表达式中声明了一个变量，那么该变量的每个子句中都将有该变量 对应的类型。在这些 case 中重用一个名字也是符合语义的，实际上是在每个 case 里声明了 一个不同类型但同名的新变量。 var t interface{} t = functionOfSomeType() switch t := t.(type) { default: fmt.Printf("unexpected type %T", t) // %T 输出 t 是什么类型 case bool: fmt.Printf("boolean %t\n", t) // t 是 bool 类型 case int: fmt.Printf("integer %d\n", t) // t 是 int 类型 case *bool: fmt.Printf("pointer to boolean %t\n", *t) // t 是 *bool 类型 case *int: fmt.Printf("pointer to integer %d\n", *t) // t 是 *int 类型 } 控制结构 31 Functions 函数 Multiple return values 多值返回 One of Go's unusual features is that functions and methods can return multiple values. This form can be used to improve on a couple of clumsy idioms in C programs: in-band error returns such as -1 for EOF and modifying an argument passed by address. Go 与众不同的特性之一就是函数和方法可返回多个值。这种形式可以改善 C 中一些笨拙的习 惯： 将错误值返回（例如用 -1 表示 EOF）和修改通过地址传入的实参。 In C, a write error is signaled by a negative count with the error code secreted away in a volatile location. In Go, Write can return a count and an error: “Yes, you wrote some bytes but not all of them because you filled the device”. The signature of the Write method on files from package os is: 在 C 中，写入操作发生的错误会用一个负数标记，而错误码会隐藏在某个不确定的位置。 而 在 Go 中，Write 会返回写入的字节数以及一个错误： “是的，您写入了一些字节，但并未全 部写入，因为设备已满”。 在 os 包中，File.Write 的签名为： func (file *File) Write(b []byte) (n int, err error) and as the documentation says, it returns the number of bytes written and a non-nil error when n != len(b). This is a common style; see the section on error handling for more examples. 正如文档所述，它返回写入的字节数，并在 n != len(b) 时返回一个非 nil 的 error 错误值。 这 是一种常见的编码风格，更多示例见错误处理一节。 A similar approach obviates the need to pass a pointer to a return value to simulate a reference parameter. Here's a simple-minded function to grab a number from a position in a byte slice, returning the number and the next position. 我们可以采用一种简单的方法。来避免为模拟引用参数而传入指针。 以下简单的函数可从字 节数组中的特定位置获取其值，并返回该数值和下一个位置。 函数 32 func nextInt(b []byte, i int) (int, int) { for ; i < len(b) && !isDigit(b[i]); i++ { } x := 0 for ; i < len(b) && isDigit(b[i]); i++ { x = x*10 + int(b[i]) - '0' } return x, i } You could use it to scan the numbers in an input slice b like this: 你可以像下面这样，通过它扫描输入的切片 b 来获取数字。 for i := 0; i < len(b); { x, i = nextInt(b, i) fmt.Println(x) } Named result parameters 可命名结果形参 The return or result "parameters" of a Go function can be given names and used as regular variables, just like the incoming parameters. When named, they are initialized to the zero values for their types when the function begins; if the function executes a return statement with no arguments, the current values of the result parameters are used as the returned values. Go 函数的返回值或结果 “形参” 可被命名，并作为常规变量使用，就像传入的形参一样。 命 名后，一旦该函数开始执行，它们就会被初始化为与其类型相应的零值； 若该函数执行了一 条不带实参的 return 语句，则结果形参的当前值将被返回。 The names are not mandatory but they can make code shorter and clearer: they're documentation. If we name the results of nextInt it becomes obvious which returned int is which. 此名称不是强制性的，但它们能使代码更加简短清晰：它们就是文档。若我们命名了 nextInt 的结果，那么它返回的 int 就值如其意了。 func nextInt(b []byte, pos int) (value, nextPos int) { Because named results are initialized and tied to an unadorned return, they can simplify as well as clarify. Here's a version of io.ReadFull that uses them well: 函数 33 由于被命名的结果已经初始化，且已经关联至无参数的返回，它们就能让代码简单而清晰。 下面的 io.ReadFull 就是个很好的例子： func ReadFull(r Reader, buf []byte) (n int, err error) { for len(buf) > 0 && err == nil { var nr int nr, err = r.Read(buf) n += nr buf = buf[nr:] } return } Defer Go's defer statement schedules a function call (the deferred function) to be run immediately before the function executing the defer returns. It's an unusual but effective way to deal with situations such as resources that must be released regardless of which path a function takes to return. The canonical examples are unlocking a mutex or closing a file. Go 的 defer 语句用于预设一个函数调用（即推迟执行函数）， 该函数会在执行 defer 的函数 返回之前立即执行。它显得非比寻常， 但却是处理一些事情的有效方式，例如无论以何种路 径返回，都必须释放资源的函数。 典型的例子就是解锁互斥和关闭文件。 // Contents returns the file's contents as a string. func Contents(filename string) (string, error) { f, err := os.Open(filename) if err != nil { return "", err } defer f.Close() // f.Close will run when we're finished. var result []byte buf := make([]byte, 100) for { n, err := f.Read(buf[0:]) result = append(result, buf[0:n]...) // append is discussed later. if err != nil { if err == io.EOF { break } return "", err // f will be closed if we return here. } } return string(result), nil // f will be closed if we return here. } 函数 34 // Contents 将文件的内容作为字符串返回。 func Contents(filename string) (string, error) { f, err := os.Open(filename) if err != nil { return "", err } defer f.Close() // f.Close 会在我们结束后运行。 var result []byte buf := make([]byte, 100) for { n, err := f.Read(buf[0:]) result = append(result, buf[0:n]...) // append 将在后面讨论。 if err != nil { if err == io.EOF { break } return "", err // 我们在这里返回后，f 就会被关闭。 } } return string(result), nil // 我们在这里返回后，f 就会被关闭。 } Deferring a call to a function such as Close has two advantages. First, it guarantees that you will never forget to close the file, a mistake that's easy to make if you later edit the function to add a new return path. Second, it means that the close sits near the open, which is much clearer than placing it at the end of the function. 推迟诸如 Close 之类的函数调用有两点好处：第一， 它能确保你不会忘记关闭文件。如果你 以后又为该函数添加了新的返回路径时， 这种情况往往就会发生。第二，它意味着 “关闭” 离 “打开” 很近， 这总比将它放在函数结尾处要清晰明了。 The arguments to the deferred function (which include the receiver if the function is a method) are evaluated when the defer executes, not when the call executes. Besides avoiding worries about variables changing values as the function executes, this means that a single deferred call site can defer multiple function executions. Here's a silly example. 被推迟函数的实参（如果该函数为方法则还包括接收者）在推迟执行时就会求值， 而不是在 调用执行时才求值。这样不仅无需担心变量值在函数执行时被改变， 同时还意味着单个已推 迟的调用可推迟多个函数的执行。下面是个简单的例子。 for i := 0; i < 5; i++ { defer fmt.Printf("%d ", i) } 函数 35 Deferred functions are executed in LIFO order, so this code will cause 4 3 2 1 0 to be printed when the function returns. A more plausible example is a simple way to trace function execution through the program. We could write a couple of simple tracing routines like this: 被推迟的函数按照后进先出（LIFO）的顺序执行，因此以上代码在函数返回时会打印 4 3 2 1 0。一个更具实际意义的例子是通过一种简单的方法， 用程序来跟踪函数的执行。我们可以编 写一对简单的跟踪例程： func trace(s string) { fmt.Println("entering:", s) } func untrace(s string) { fmt.Println("leaving:", s) } // Use them like this: func a() { trace("a") defer untrace("a") // do something.... } func trace(s string) { fmt.Println("entering:", s) } func untrace(s string) { fmt.Println("leaving:", s) } // 像这样使用它们： func a() { trace("a") defer untrace("a") // 做一些事情.... } We can do better by exploiting the fact that arguments to deferred functions are evaluated when the defer executes. The tracing routine can set up the argument to the untracing routine. This example: 我们可以充分利用这个特点，即被推迟函数的实参在 defer 执行时才会被求值。 跟踪例程可 针对反跟踪例程设置实参。以下例子： 函数 36 func trace(s string) string { fmt.Println("entering:", s) return s } func un(s string) { fmt.Println("leaving:", s) } func a() { defer un(trace("a")) fmt.Println("in a") } func b() { defer un(trace("b")) fmt.Println("in b") a() } func main() { b() } prints 会打印 entering: b in b entering: a in a leaving: a leaving: b For programmers accustomed to block-level resource management from other languages, defer may seem peculiar, but its most interesting and powerful applications come precisely from the fact that it's not block-based but function-based. In the section on panic and recover we'll see another example of its possibilities. 对于习惯其它语言中块级资源管理的程序员，defer 似乎有点怪异， 但它最有趣而强大的应用 恰恰来自于其基于函数而非块的特点。在 panic 和 recover 这两节中，我们将看到关于它可能 性的其它例子。 函数 37 Data 数据 Allocation with new new 分配 Go has two allocation primitives, the built-in functions new and make. They do different things and apply to different types, which can be confusing, but the rules are simple. Let's talk about new first. It's a built-in function that allocates memory, but unlike its namesakes in some other languages it does not initialize the memory, it only zeros it. That is, new(T) allocates zeroed storage for a new item of type T and returns its address, a value of type *T . In Go terminology, it returns a pointer to a newly allocated zero value of type T. Go 提供了两种分配原语，即内建函数 new 和 make。 它们所做的事情不同，所应用的类型 也不同。它们可能会引起混淆，但规则却很简单。 让我们先来看看 new。这是个用来分配内 存的内建函数， 但与其它语言中的同名函数不同，它不会初始化内存，只会将内存置零。 也 就是说，new(T) 会为类型为 T 的新项分配已置零的内存空间， 并返回它的地址，也就是一个 类型为 *T 的值。用 Go 的术语来说，它返回一个指针， 该指针指向新分配的，类型为 T 的 零值。 Since the memory returned by new is zeroed, it's helpful to arrange when designing your data structures that the zero value of each type can be used without further initialization. This means a user of the data structure can create one with new and get right to work. For example, the documentation for bytes.Buffer states that "the zero value for Buffer is an empty buffer ready to use." Similarly, sync.Mutex does not have an explicit constructor or Init method. Instead, the zero value for a sync.Mutex is defined to be an unlocked mutex. 既然 new 返回的内存已置零，那么当你设计数据结构时， 每种类型的零值就不必进一步初始 化了，这意味着该数据结构的使用者只需用 new 创建一个新的对象就能正常工作。例如， bytes.Buffer 的文档中提到 “零值的 Buffer 就是已准备就绪的缓冲区。" 同样，sync.Mutex 并 没有显式的构造函数或 Init 方法， 而是零值的 sync.Mutex 就已经被定义为已解锁的互斥锁 了。 The zero-value-is-useful property works transitively. Consider this type declaration. “零值属性” 可以带来各种好处。考虑以下类型声明。 数据 38 type SyncedBuffer struct { lock sync.Mutex buffer bytes.Buffer } Values of type SyncedBuffer are also ready to use immediately upon allocation or just declaration. In the next snippet, both p and v will work correctly without further arrangement. SyncedBuffer 类型的值也是在声明时就分配好内存就绪了。后续代码中， p 和 v 无需进一步 处理即可正确工作。 p := new(SyncedBuffer) // type *SyncedBuffer var v SyncedBuffer // type SyncedBuffer Constructors and composite literals 构造函数与复合字面 Sometimes the zero value isn't good enough and an initializing constructor is necessary, as in this example derived from package os. 有时零值还不够好，这时就需要一个初始化构造函数，如来自 os 包中的这段代码所示。 func NewFile(fd int, name string) *File { if fd < 0 { return nil } f := new(File) f.fd = fd f.name = name f.dirinfo = nil f.nepipe = 0 return f } There's a lot of boiler plate in there. We can simplify it using a composite literal, which is an expression that creates a new instance each time it is evaluated. 这里显得代码过于冗长。我们可通过复合字面来简化它， 该表达式在每次求值时都会创建新 的实例。 数据 39 func NewFile(fd int, name string) *File { if fd < 0 { return nil } f := File{fd, name, nil, 0} return &f } Note that, unlike in C, it's perfectly OK to return the address of a local variable; the storage associated with the variable survives after the function returns. In fact, taking the address of a composite literal allocates a fresh instance each time it is evaluated, so we can combine these last two lines. 请注意，返回一个局部变量的地址完全没有问题，这点与 C 不同。该局部变量对应的数据 在 函数返回后依然有效。实际上，每当获取一个复合字面的地址时，都将为一个新的实例分配 内存， 因此我们可以将上面的最后两行代码合并： return &File{fd, name, nil, 0} The fields of a composite literal are laid out in order and must all be present. However, by labeling the elements explicitly as field:value pairs, the initializers can appear in any order, with the missing ones left as their respective zero values. Thus we could say 复合字面的字段必须按顺序全部列出。但如果以 字段: 值 对的形式明确地标出元素，初始化 字段时就可以按任何顺序出现，未给出的字段值将赋予零值。 因此，我们可以用如下形式： return &File{fd: fd, name: name} As a limiting case, if a composite literal contains no fields at all, it creates a zero value for the type. The expressions new(File) and &File{} are equivalent. 少数情况下，若复合字面不包括任何字段，它将创建该类型的零值。表达式 new(File) 和 &File{} 是等价的。 Composite literals can also be created for arrays, slices, and maps, with the field labels being indices or map keys as appropriate. In these examples, the initializations work regardless of the values of Enone, Eio, and Einval, as long as they are distinct. 复合字面同样可用于创建数组、切片以及映射，字段标签是索引还是映射键则视情况而定。 在下例初始化过程中，无论 Enone、Eio 和 Einval 的值是什么，只要它们的标签不同就行。 数据 40 a := [...]string {Enone: "no error", Eio: "Eio", Einval: "invalid argument"} s := []string {Enone: "no error", Eio: "Eio", Einval: "invalid argument"} m := map[int]string{Enone: "no error", Eio: "Eio", Einval: "invalid argument"} Allocation with make make 分配 Back to allocation. The built-in function make(T, args) serves a purpose different from new(T). It creates slices, maps, and channels only, and it returns an initialized (not zeroed) value of type T (not *T ). The reason for the distinction is that these three types represent, under the covers, references to data structures that must be initialized before use. A slice, for example, is a three-item descriptor containing a pointer to the data (inside an array), the length, and the capacity, and until those items are initialized, the slice is nil. For slices, maps, and channels, make initializes the internal data structure and prepares the value for use. For instance, 再回到内存分配上来。内建函数 make(T, args) 的目的不同于 new(T)。它只用于创建切片、 映射和信道，并返回类型为 T（而非 *T ）的一个已初始化 （而非置零）的值。 出现这种用 差异的原因在于，这三种类型本质上为引用数据类型，它们在使用前必须初始化。 例如，切 片是一个具有三项内容的描述符，包含一个指向（数组内部）数据的指针、长度以及容量， 在这三项被初始化之前，该切片为 nil。对于切片、映射和信道，make 用于初始化其内部的 数据结构并准备好将要使用的值。例如， make([]int, 10, 100) allocates an array of 100 ints and then creates a slice structure with length 10 and a capacity of 100 pointing at the first 10 elements of the array. (When making a slice, the capacity can be omitted; see the section on slices for more information.) In contrast, new([]int) returns a pointer to a newly allocated, zeroed slice structure, that is, a pointer to a nil slice value. 会分配一个具有 100 个 int 的数组空间，接着创建一个长度为 10， 容量为 100 并指向该数组 中前 10 个元素的切片结构。（生成切片时，其容量可以省略，更多信息见切片一节。） 与 此相反，new([]int) 会返回一个指向新分配的，已置零的切片结构， 即一个指向 nil 切片值的 指针。 These examples illustrate the difference between new and make. 下面的例子阐明了 new 和 make 之间的区别： 数据 41 var p *[]int = new([]int) // allocates slice structure; *p == nil; rarely useful var v []int = make([]int, 100) // the slice v now refers to a new array of 100 ints // Unnecessarily complex: var p *[]int = new([]int) *p = make([]int, 100, 100) // Idiomatic: v := make([]int, 100) var p *[]int = new([]int) // 分配切片结构；*p == nil；基本没用 var v []int = make([]int, 100) // 切片 v 现在引用了一个具有 100 个 int 元素的新数组 // 没必要的复杂： var p *[]int = new([]int) *p = make([]int, 100, 100) // 习惯用法： v := make([]int, 100) Remember that make applies only to maps, slices and channels and does not return a pointer. To obtain an explicit pointer allocate with new or take the address of a variable explicitly. 请记住，make 只适用于映射、切片和信道且不返回指针。若要获得明确的指针， 请使用 new 分配内存。 Arrays 数组 Arrays are useful when planning the detailed layout of memory and sometimes can help avoid allocation, but primarily they are a building block for slices, the subject of the next section. To lay the foundation for that topic, here are a few words about arrays. 在详细规划内存布局时，数组是非常有用的，有时还能避免过多的内存分配， 但它们主要用 作切片的构件。这是下一节的主题了，不过要先说上几句来为它做铺垫。 There are major differences between the ways arrays work in Go and C. In Go, Arrays are values. Assigning one array to another copies all the elements. In particular, if you pass an array to a function, it will receive a copy of the array, not a pointer to it. The size of an array is part of its type. The types [10]int and [20]int are distinct. 数据 42 以下为数组在 Go 和 C 中的主要区别。在 Go 中， 数组是值。将一个数组赋予另一个数组会复制其所有元素。 特别地，若将某个数组传入某个函数，它将接收到该数组的一份副本而非指针。 数组的大小是其类型的一部分。类型 [10]int 和 [20]int 是不同的。 The value property can be useful but also expensive; if you want C-like behavior and efficiency, you can pass a pointer to the array. 数组为值的属性很有用，但代价高昂；若你想要 C 那样的行为和效率，你可以传递一个指向 该数组的指针。 func Sum(a *[3]float64) (sum float64) { for _, v := range *a { sum += v } return } array := [...]float64{7.0, 8.5, 9.1} x := Sum(&array) // Note the explicit address-of operator func Sum(a *[3]float64) (sum float64) { for _, v := range *a { sum += v } return } array := [...]float64{7.0, 8.5, 9.1} x := Sum(&array) // 注意显式的取址操作 But even this style isn't idiomatic Go. Use slices instead. 但这并不是 Go 的习惯用法，切片才是。 Slices 切片 Slices wrap arrays to give a more general, powerful, and convenient interface to sequences of data. Except for items with explicit dimension such as transformation matrices, most array programming in Go is done with slices rather than simple arrays. 切片通过对数组进行封装，为数据序列提供了更通用、强大而方便的接口。 除了矩阵变换这 类需要明确维度的情况外，Go 中的大部分数组编程都是通过切片来完成的。 数据 43 Slices hold references to an underlying array, and if you assign one slice to another, both refer to the same array. If a function takes a slice argument, changes it makes to the elements of the slice will be visible to the caller, analogous to passing a pointer to the underlying array. A Read function can therefore accept a slice argument rather than a pointer and a count; the length within the slice sets an upper limit of how much data to read. Here is the signature of the Read method of the File type in package os: 切片保存了对底层数组的引用，若你将某个切片赋予另一个切片，它们会引用同一个数组。 若某个函数将一个切片作为参数传入，则它对该切片元素的修改对调用者而言同样可见， 这 可以理解为传递了底层数组的指针。因此，Read 函数可接受一个切片实参 而非一个指针和 一个计数；切片的长度决定了可读取数据的上限。以下为 os 包中 File 类型的 Read 方法签 名: func (file *File) Read(buf []byte) (n int, err error) The method returns the number of bytes read and an error value, if any. To read into the first 32 bytes of a larger buffer buf, slice (here used as a verb) the buffer. 该方法返回读取的字节数和一个错误值（若有的话）。若要从更大的缓冲区 b 中读取前 32 个 字节，只需对其进行切片即可。 n, err := f.Read(buf[0:32]) Such slicing is common and efficient. In fact, leaving efficiency aside for the moment, the following snippet would also read the first 32 bytes of the buffer. 这种切片的方法常用且高效。若不谈效率，以下片段同样能读取该缓冲区的前 32 个字节。 var n int var err error for i := 0; i < 32; i++ { nbytes, e := f.Read(buf[i:i+1]) // Read one byte. if nbytes == 0 || e != nil { err = e break } n += nbytes } 数据 44 var n int var err error for i := 0; i < 32; i++ { nbytes, e := f.Read(buf[i:i+1]) // 读取一个字节 if nbytes == 0 || e != nil { err = e break } n += nbytes } The length of a slice may be changed as long as it still fits within the limits of the underlying array; just assign it to a slice of itself. The capacity of a slice, accessible by the built-in function cap, reports the maximum length the slice may assume. Here is a function to append data to a slice. If the data exceeds the capacity, the slice is reallocated. The resulting slice is returned. The function uses the fact that len and cap are legal when applied to the nil slice, and return 0. 只要切片不超出底层数组的限制，它的长度就是可变的，只需将它赋予其自身的切片即可。 切片的容量可通过内建函数 cap 获得，它将给出该切片可取得的最大长度。 以下是将数据追 加到切片的函数。若数据超出其容量，则会重新分配该切片。返回值即为所得的切片。 该函 数中所使用的 len 和 cap 在应用于 nil 切片时是合法的，它会返回 0. func Append(slice, data[]byte) []byte { l := len(slice) if l + len(data) > cap(slice) { // reallocate // Allocate double what's needed, for future growth. newSlice := make([]byte, (l+len(data))*2) // The copy function is predeclared and works for any slice type. copy(newSlice, slice) slice = newSlice } slice = slice[0:l+len(data)] for i, c := range data { slice[l+i] = c } return slice } 数据 45 func Append(slice, data[]byte) []byte { l := len(slice) if l + len(data) > cap(slice) { // 重新分配 // 为了后面的增长，需分配两份。 newSlice := make([]byte, (l+len(data))*2) // copy 函数是预声明的，且可用于任何切片类型。 copy(newSlice, slice) slice = newSlice } slice = slice[0:l+len(data)] for i, c := range data { slice[l+i] = c } return slice } We must return the slice afterwards because, although Append can modify the elements of slice, the slice itself (the run-time data structure holding the pointer, length, and capacity) is passed by value. 最终我们必须返回切片，因为尽管 Append 可修改 slice 的元素，但切片自身（其运行时数据 结构包含指针、长度和容量）是通过值传递的。 The idea of appending to a slice is so useful it's captured by the append built-in function. To understand that function's design, though, we need a little more information, so we'll return to it later. 向切片追加东西的想法非常有用，因此有专门的内建函数 append。 要理解该函数的设计，我 们还需要一些额外的信息，我们将稍后再介绍它。 Two-dimensional slices 二维切片 Go's arrays and slices are one-dimensional. To create the equivalent of a 2D array or slice, it is necessary to define an array-of-arrays or slice-of-slices, like this: Go 的数组和切片都是一维的。要创建等价的二维数组或切片，就必须定义一个数组的数组， 或切片的切片，就像这样： type Transform [3][3]float64 // A 3x3 array, really an array of arrays. type LinesOfText [][]byte // A slice of byte slices. 数据 46 type Transform [3][3]float64 // 一个 3x3 的数组，其实是包含多个数组的一个数组。 type LinesOfText [][]byte // 包含多个字节切片的一个切片。 Because slices are variable-length, it is possible to have each inner slice be a different length. That can be a common situation, as in our LinesOfText example: each line has an independent length. 由于切片长度是可变的，因此其内部可能拥有多个不同长度的切片。在我们的 LinesOfText 例 子中，这是种常见的情况：每行都有其自己的长度。 text := LinesOfText{ []byte("Now is the time"), []byte("for all good gophers"), []byte("to bring some fun to the party."), } Sometimes it's necessary to allocate a 2D slice, a situation that can arise when processing scan lines of pixels, for instance. There are two ways to achieve this. One is to allocate each slice independently; the other is to allocate a single array and point the individual slices into it. Which to use depends on your application. If the slices might grow or shrink, they should be allocated independently to avoid overwriting the next line; if not, it can be more efficient to construct the object with a single allocation. For reference, here are sketches of the two methods. First, a line at a time: 有时必须分配一个二维数组，例如在处理像素的扫描行时，这种情况就会发生。 我们有两种 方式来达到这个目的。一种就是独立地分配每一个切片；而另一种就是只分配一个数组， 将 各个切片都指向它。采用哪种方式取决于你的应用。若切片会增长或收缩， 就应该通过独立 分配来避免覆盖下一行；若不会，用单次分配来构造对象会更加高效。 以下是这两种方法的 大概代码，仅供参考。首先是一次一行的： // Allocate the top-level slice. picture := make([][]uint8, YSize) // One row per unit of y. // Loop over the rows, allocating the slice for each row. for i := range picture { picture[i] = make([]uint8, XSize) } // 分配顶层切片。 picture := make([][]uint8, YSize) // 每 y 个单元一行。 // 遍历行，为每一行都分配切片 for i := range picture { picture[i] = make([]uint8, XSize) } 数据 47 And now as one allocation, sliced into lines: 现在是一次分配，对行进行切片： // Allocate the top-level slice, the same as before. picture := make([][]uint8, YSize) // One row per unit of y. // Allocate one large slice to hold all the pixels. pixels := make([]uint8, XSize*YSize) // Has type []uint8 even though picture is [][]ui nt8. // Loop over the rows, slicing each row from the front of the remaining pixels slice. for i := range picture { picture[i], pixels = pixels[:XSize], pixels[XSize:] } // 分配顶层切片，和前面一样。 picture := make([][]uint8, YSize) // 每 y 个单元一行。 // 分配一个大的切片来保存所有像素 pixels := make([]uint8, XSize*YSize) // 拥有类型 []uint8，尽管图片是 [][]uint8. // 遍历行，从剩余像素切片的前面切出每行来。 for i := range picture { picture[i], pixels = pixels[:XSize], pixels[XSize:] } Maps 映射 Maps are a convenient and powerful built-in data structure that associate values of one type (the key) with values of another type (the element or value) The key can be of any type for which the equality operator is defined, such as integers, floating point and complex numbers, strings, pointers, interfaces (as long as the dynamic type supports equality), structs and arrays. Slices cannot be used as map keys, because equality is not defined on them. Like slices, maps hold references to an underlying data structure. If you pass a map to a function that changes the contents of the map, the changes will be visible in the caller. 映射是方便而强大的内建数据结构，它可以关联不同类型的值。其键可以是任何相等性操作 符支持的类型， 如整数、浮点数、复数、字符串、指针、接口（只要其动态类型支持相等性 判断）、结构以及数组。 切片不能用作映射键，因为它们的相等性还未定义。与切片一样， 映射也是引用类型。 若将映射传入函数中，并更改了该映射的内容，则此修改对调用者同样 可见。 Maps can be constructed using the usual composite literal syntax with colon-separated key- value pairs, so it's easy to build them during initialization. 数据 48 映射可使用一般的复合字面语法进行构建，其键 - 值对使用逗号分隔，因此可在初始化时很容 易地构建它们。 var timeZone = map[string]int{ "UTC": 0*60*60, "EST": -5*60*60, "CST": -6*60*60, "MST": -7*60*60, "PST": -8*60*60, } Assigning and fetching map values looks syntactically just like doing the same for arrays and slices except that the index doesn't need to be an integer. 赋值和获取映射值的语法类似于数组，不同的是映射的索引不必为整数。 offset := timeZone["EST"] An attempt to fetch a map value with a key that is not present in the map will return the zero value for the type of the entries in the map. For instance, if the map contains integers, looking up a non-existent key will return 0. A set can be implemented as a map with value type bool. Set the map entry to true to put the value in the set, and then test it by simple indexing. 若试图通过映射中不存在的键来取值，就会返回与该映射中项的类型对应的零值。 例如，若 某个映射包含整数，当查找一个不存在的键时会返回 0。 集合可实现成一个值类型为 bool 的 映射。将该映射中的项置为 true 可将该值放入集合中，此后通过简单的索引操作即可判断是 否存在。 attended := map[string]bool{ "Ann": true, "Joe": true, ... } if attended[person] { // will be false if person is not in the map fmt.Println(person, "was at the meeting") } 数据 49 attended := map[string]bool{ "Ann": true, "Joe": true, ... } if attended[person] { // 若某人不在此映射中，则为 false fmt.Println(person, "正在开会") } Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that the empty string because it's not in the map at all? You can discriminate with a form of multiple assignment. 有时你需要区分某项是不存在还是其值为零值。如对于一个值本应为零的 "UTC" 条目，也可 能是由于不存在该项而得到零值。你可以使用多重赋值的形式来分辨这种情况。 var seconds int var ok bool seconds, ok = timeZone[tz] For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together with a nice error report: 显然，我们可称之为 “逗号 ok” 惯用法。在下面的例子中，若 tz 存在， seconds 就会被赋予 适当的值，且 ok 会被置为 true； 若不存在，seconds 则会被置为零，而 ok 会被置为 false。 func offset(tz string) int { if seconds, ok := timeZone[tz]; ok { return seconds } log.Println("unknown time zone:", tz) return 0 } To test for presence in the map without worrying about the actual value, you can use the blank identifier ( _ ) in place of the usual variable for the value. 若仅需判断映射中是否存在某项而不关心实际的值，可使用 空白标识符 （ _ ）来代替该值的 一般变量。 _, present := timeZone[tz] 数据 50 To delete a map entry, use the delete built-in function, whose arguments are the map and the key to be deleted. It's safe to do this even if the key is already absent from the map. 要删除映射中的某项，可使用内建函数 delete，它以映射及要被删除的键为实参。 即便对应 的键不在该映射中，此操作也是安全的。 delete(timeZone, "PDT") // Now on Standard Time delete(timeZone, "PDT") // 现在用标准时间 Printing 打印 Formatted printing in Go uses a style similar to C's printf family but is richer and more general. The functions live in the fmt package and have capitalized names: fmt.Printf, fmt.Fprintf, fmt.Sprintf and so on. The string functions (Sprintf etc.) return a string rather than filling in a provided buffer. Go 采用的格式化打印风格和 C 的 printf 族类似，但却更加丰富而通用。 这些函数位于 fmt 包 中，且函数名首字母均为大写：如 fmt.Printf、fmt.Fprintf，fmt.Sprintf 等。 字符串函数 （Sprintf 等）会返回一个字符串，而非填充给定的缓冲区。 You don't need to provide a format string. For each of Printf, Fprintf and Sprintf there is another pair of functions, for instance Print and Println. These functions do not take a format string but instead generate a default format for each argument. The Println versions also insert a blank between arguments and append a newline to the output while the Print versions add blanks only if the operand on neither side is a string. In this example each line produces the same output. 你无需提供一个格式字符串。每个 Printf、Fprintf 和 Sprintf 都分别对应另外的函数，如 Print 与 Println。 这些函数并不接受格式字符串，而是为每个实参生成一种默认格式。Println 系列 的函数还会在实参中插入空格，并在输出时追加一个换行符，而 Print 版本仅在操作数两侧都 没有字符串时才添加空白。以下示例中各行产生的输出都是一样的。 fmt.Printf("Hello %d\n", 23) fmt.Fprint(os.Stdout, "Hello ", 23, "\n") fmt.Println("Hello", 23) fmt.Println(fmt.Sprint("Hello ", 23)) 数据 51 The formatted print functions fmt.Fprint and friends take as a first argument any object that implements the io.Writer interface; the variables os.Stdout and os.Stderr are familiar instances. fmt.Fprint 一类的格式化打印函数可接受任何实现了 io.Writer 接口的对象作为第一个实参；变 量 os.Stdout 与 os.Stderr 都是人们熟知的例子。 Here things start to diverge from C. First, the numeric formats such as %d do not take flags for signedness or size; instead, the printing routines use the type of the argument to decide these properties. 从这里开始，就与 C 有些不同了。首先，像 %d 这样的数值格式并不接受表示符号或大小的 标记， 打印例程会根据实参的类型来决定这些属性。 var x uint64 = 1<<64 - 1 fmt.Printf("%d %x; %d %x\n", x, x, int64(x), int64(x)) prints 将打印 18446744073709551615 ffffffffffffffff; -1 -1 If you just want the default conversion, such as decimal for integers, you can use the catchall format %v (for “value”); the result is exactly what Print and Println would produce. Moreover, that format can print any value, even arrays, slices, structs, and maps. Here is a print statement for the time zone map defined in the previous section. 若你只想要默认的转换，如使用十进制的整数，你可以使用通用的格式 %v（对应 “值”）；其 结果与 Print 和 Println 的输出完全相同。此外，这种格式还能打印任意值，甚至包括数组、 结构体和映射。 以下是打印上一节中定义的时区映射的语句。 fmt.Printf("%v\n", timeZone) // or just fmt.Println(timeZone) which gives output map[CST:-21600 PST:-28800 EST:-18000 UTC:0 MST:-25200] fmt.Printf("%v\n", timeZone) // 或只用 fmt.Println(timeZone) 这会输出 数据 52 map[CST:-21600 PST:-28800 EST:-18000 UTC:0 MST:-25200] For maps the keys may be output in any order, of course. When printing a struct, the modified format %+v annotates the fields of the structure with their names, and for any value the alternate format %#v prints the value in full Go syntax. 当然，映射中的键可能按任意顺序输出。当打印结构体时，改进的格式 %+v 会为结构体的每 个字段添上字段名，而另一种格式 %#v 将完全按照 Go 的语法打印值。 type T struct { a int b float64 c string } t := &T{ 7, -2.35, "abc\tdef" } fmt.Printf("%v\n", t) fmt.Printf("%+v\n", t) fmt.Printf("%#v\n", t) fmt.Printf("%#v\n", timeZone) prints 将打印 &{7 -2.35 abc def} &{a:7 b:-2.35 c:abc def} &main.T{a:7, b:-2.35, c:"abc\tdef"} map[string] int{"CST":-21600, "PST":-28800, "EST":-18000, "UTC":0, "MST":-25200} (Note the ampersands.) That quoted string format is also available through %q when applied to a value of type string or []byte. The alternate format %#q will use backquotes instead if possible. (The %q format also applies to integers and runes, producing a single-quoted rune constant.) Also, %x works on strings, byte arrays and byte slices as well as on integers, generating a long hexadecimal string, and with a space in the format (% x) it puts spaces between the bytes. （请注意其中的 & 符号）当遇到 string 或 []byte 值时， 可使用 %q 产生带引号的字符串；而 格式 %#q 会尽可能使用反引号。 （%q 格式也可用于整数和符文，它会产生一个带单引号的 符文常量。） 此外，%x 还可用于字符串、字节数组以及整数，并生成一个很长的十六进制 字符串， 而带空格的格式（% x）还会在字节之间插入空格。 Another handy format is %T, which prints the type of a value. 另一种实用的格式是 %T，它会打印某个值的类型. 数据 53 fmt.Printf("%T\n", timeZone) prints 会打印 map[string] int If you want to control the default format for a custom type, all that's required is to define a method with the signature String() string on the type. For our simple type T, that might look like this. 若你想控制自定义类型的默认格式，只需为该类型定义一个具有 String() string 签名的方法。 对于我们简单的类型 T，可进行如下操作。 func (t *T) String() string { return fmt.Sprintf("%d/%g/%q", t.a, t.b, t.c) } fmt.Printf("%v\n", t) to print in the format 会打印出如下格式： 7/-2.35/"abc\tdef" (If you need to print values of type T as well as pointers to T, the receiver for String must be of value type; this example used a pointer because that's more efficient and idiomatic for struct types. See the section below on pointers vs. value receivers for more information.) （如果你需要像指向 T 的指针那样打印类型 T 的值， String 的接收者就必须是值类型的；上 面的例子中接收者是一个指针， 因为这对结构来说更高效而通用。更多详情见 指针 vs. 值接 收者 一节.） Our String method is able to call Sprintf because the print routines are fully reentrant and can be wrapped this way. There is one important detail to understand about this approach, however: don't construct a String method by calling Sprintf in a way that will recur into your String method indefinitely. This can happen if the Sprintf call attempts to print the receiver directly as a string, which in turn will invoke the method again. It's a common and easy mistake to make, as this example shows. 我们的 String 方法也可调用 Sprintf， 因为打印例程可以完全重入并按这种方式封装。不过要 理解这种方式，还有一个重要的细节： 请勿通过调用 Sprintf 来构造 String 方法，因为它会无 限递归你的的 String 方法。 数据 54 type MyString string func (m MyString) String() string { return fmt.Sprintf("MyString=%s", m) // Error: will recur forever. } type MyString string func (m MyString) String() string { return fmt.Sprintf("MyString=%s", m) // 错误：会无限递归 } It's also easy to fix: convert the argument to the basic string type, which does not have the method. 要解决这个问题也很简单：将该实参转换为基本的字符串类型，它没有这个方法。 type MyString string func (m MyString) String() string { return fmt.Sprintf("MyString=%s", string(m)) // OK: note conversion. } type MyString string func (m MyString) String() string { return fmt.Sprintf("MyString=%s", string(m)) // 可以：注意转换 } In the initialization section we'll see another technique that avoids this recursion. 在 初始化 一节中，我们将看到避免这种递归的另一种技术。 Another printing technique is to pass a print routine's arguments directly to another such routine. The signature of Printf uses the type ...interface{} for its final argument to specify that an arbitrary number of parameters (of arbitrary type) can appear after the format. 另一种打印技术就是将打印例程的实参直接传入另一个这样的例程。Printf 的签名为其最后的 实参使用了 ...interface{} 类型，这样格式的后面就能出现任意数量，任意类型的形参了。 func Printf(format string, v ...interface{}) (n int, err error) { Within the function Printf, v acts like a variable of type []interface{} but if it is passed to another variadic function, it acts like a regular list of arguments. Here is the implementation of the function log.Println we used above. It passes its arguments directly to fmt.Sprintln for 数据 55 the actual formatting. 在 Printf 函数中，v 看起来更像是 []interface{} 类型的变量，但如果将它传递到另一个变参函 数中，它就像是常规实参列表了。 以下是我们之前用过的 log.Println 的实现。它直接将其实 参传递给 fmt.Sprintln 进行实际的格式化。 // Println prints to the standard logger in the manner of fmt.Println. func Println(v ...interface{}) { std.Output(2, fmt.Sprintln(v...)) // Output takes parameters (int, string) } // Println 通过 fmt.Println 的方式将日志打印到标准记录器。 func Println(v ...interface{}) { std.Output(2, fmt.Sprintln(v...)) // Output 接受形参 (int, string) } We write ... after v in the nested call to Sprintln to tell the compiler to treat v as a list of arguments; otherwise it would just pass v as a single slice argument. 在该 Sprintln 嵌套调用中，我们将 ... 写在 v 之后来告诉编译器将 v 视作一个实参列表，否则 它会将 v 当做单一的切片实参来传递。 There's even more to printing than we've covered here. See the godoc documentation for package fmt for the details. 还有很多关于打印知识点没有提及。详情请参阅 godoc 对 fmt 包的说明文档。 By the way, a ... parameter can be of a specific type, for instance ...int for a min function that chooses the least of a list of integers: 顺便一提，... 形参可指定具体的类型，例如从整数列表中选出最小值的函数 min，其形参可 为 ...int 类型。 func Min(a ...int) int { min := int(^uint(0) >> 1) // largest int for _, i := range a { if i < min { min = i } } return min } 数据 56 func Min(a ...int) int { min := int(^uint(0) >> 1) // 最大的 int for _, i := range a { if i < min { min = i } } return min } Append 追加 Now we have the missing piece we needed to explain the design of the append built-in function. The signature of append is different from our custom Append function above. Schematically, it's like this: 现在我们要对内建函数 append 的设计进行补充说明。append 函数的签名不同于前面我们自 定义的 Append 函数。大致来说，它就像这样： func append(slice []T, elements ...T) []T where T is a placeholder for any given type. You can't actually write a function in Go where the type T is determined by the caller. That's why append is built in: it needs support from the compiler. 其中的 T 为任意给定类型的占位符。实际上，你无法在 Go 中编写一个类型 T 由调用者决定 的函数。这也就是为何 append 为内建函数的原因：它需要编译器的支持。 What append does is append the elements to the end of the slice and return the result. The result needs to be returned because, as with our hand-written Append, the underlying array may change. This simple example append 会在切片末尾追加元素并返回结果。我们必须返回结果， 原因与我们手写的 Append 一样，即底层数组可能会被改变。以下简单的例子 x := []int{1,2,3} x = append(x, 4, 5, 6) fmt.Println(x) prints [1 2 3 4 5 6]. So append works a little like Printf, collecting an arbitrary number of arguments. 数据 57 将打印 [1 2 3 4 5 6]。因此 append 有点像 Printf 那样，可接受任意数量的实参。 But what if we wanted to do what our Append does and append a slice to a slice? Easy: use ... at the call site, just as we did in the call to Output above. This snippet produces identical output to the one above. 但如果我们要像 Append 那样将一个切片追加到另一个切片中呢？ 很简单：在调用的地方使 用 ...，就像我们在上面调用 Output 那样。以下代码片段的输出与上一个相同。 x := []int{1,2,3} y := []int{4,5,6} x = append(x, y...) fmt.Println(x) Without that ..., it wouldn't compile because the types would be wrong; y is not of type int. 如果没有 ...，它就会由于类型错误而无法编译，因为 y 不是 int 类型的。 数据 58 Initialization 初始化 Although it doesn't look superficially very different from initialization in C or C++, initialization in Go is more powerful. Complex structures can be built during initialization and the ordering issues among initialized objects, even among different packages, are handled correctly. 尽管从表面上看，Go 的初始化过程与 C 或 C++ 相比并无太大差别，但它确实更为强大。 在 初始化过程中，不仅可以构建复杂的结构，还能正确处理不同包对象间的初始化顺序。 Constants 常量 Constants in Go are just that—constant. They are created at compile time, even when defined as locals in functions, and can only be numbers, characters (runes), strings or booleans. Because of the compile-time restriction, the expressions that define them must be constant expressions, evaluatable by the compiler. For instance, 1<<3 is a constant expression, while math.Sin(math.Pi/4) is not because the function call to math.Sin needs to happen at run time. Go 中的常量就是不变量。它们在编译时创建，即便它们可能是函数中定义的局部变量。 常量 只能是数字、字符（符文）、字符串或布尔值。由于编译时的限制， 定义它们的表达式必须 也是可被编译器求值的常量表达式。例如 1<<3 就是一个常量表达式，而 math.Sin(math.Pi/4) 则不是，因为对 math.Sin 的函数调用在运行时才会发生。 In Go, enumerated constants are created using the iota enumerator. Since iota can be part of an expression and expressions can be implicitly repeated, it is easy to build intricate sets of values. 在 Go 中，枚举常量使用枚举器 iota 创建。由于 iota 可为表达式的一部分，而表达式可以被 隐式地重复，这样也就更容易构建复杂的值的集合了。 初始化 59 type ByteSize float64 const ( // 通过赋予空白标识符来忽略第一个值 _ = iota // ignore first value by assigning to blank identifier KB ByteSize = 1 << (10 * iota) MB GB TB PB EB ZB YB ) The ability to attach a method such as String to any user-defined type makes it possible for arbitrary values to format themselves automatically for printing. Although you'll see it most often applied to structs, this technique is also useful for scalar types such as floating-point types like ByteSize. 由于可将 String 之类的方法附加在用户定义的类型上， 因此它就为打印时自动格式化任意值 提供了可能性，即便是作为一个通用类型的一部分。 尽管你常常会看到这种技术应用于结构 体，但它对于像 ByteSize 之类的浮点数标量等类型也是有用的。 func (b ByteSize) String() string { switch { case b >= YB: return fmt.Sprintf("%.2fYB", b/YB) case b >= ZB: return fmt.Sprintf("%.2fZB", b/ZB) case b >= EB: return fmt.Sprintf("%.2fEB", b/EB) case b >= PB: return fmt.Sprintf("%.2fPB", b/PB) case b >= TB: return fmt.Sprintf("%.2fTB", b/TB) case b >= GB: return fmt.Sprintf("%.2fGB", b/GB) case b >= MB: return fmt.Sprintf("%.2fMB", b/MB) case b >= KB: return fmt.Sprintf("%.2fKB", b/KB) } return fmt.Sprintf("%.2fB", b) } The expression YB prints as 1.00YB, while ByteSize(1e13) prints as 9.09TB. 初始化 60 表达式 YB 会打印出 1.00YB，而 ByteSize(1e13) 则会打印出 9.09TB。 The use here of Sprintf to implement ByteSize's String method is safe (avoids recurring indefinitely) not because of a conversion but because it calls Sprintf with %f, which is not a string format: Sprintf will only call the String method when it wants a string, and %f wants a floating-point value. 在这里用 Sprintf 实现 ByteSize 的 String 方法很安全（不会无限递归），这倒不是因为类型 转换，而是它以 %f 调用了 Sprintf，它并不是一种字符串格式：Sprintf 只会在它需要字符串 时才调用 String 方法，而 %f 需要一个浮点数值。 Variables 变量 Variables can be initialized just like constants but the initializer can be a general expression computed at run time. 变量的初始化与常量类似，但其初始值也可以是在运行时才被计算的一般表达式。 var ( home = os.Getenv("HOME") user = os.Getenv("USER") gopath = os.Getenv("GOPATH") ) The init function init 函数 Finally, each source file can define its own niladic init function to set up whatever state is required. (Actually each file can have multiple init functions.) And finally means finally: init is called after all the variable declarations in the package have evaluated their initializers, and those are evaluated only after all the imported packages have been initialized. 最后，每个源文件都可以通过定义自己的无参数 init 函数来设置一些必要的状态。 （其实每 个文件都可以拥有多个 init 函数。）而它的结束就意味着初始化结束： 只有该包中的所有变 量声明都通过它们的初始化器求值后 init 才会被调用， 而那些 init 只有在所有已导入的包都 被初始化后才会被求值。 Besides initializations that cannot be expressed as declarations, a common use of init functions is to verify or repair correctness of the program state before real execution begins. 初始化 61 除了那些不能被表示成声明的初始化外，init 函数还常被用在程序真正开始执行前，检验或校 正程序的状态。 func init() { if user == "" { log.Fatal("$USER not set") } if home == "" { home = "/home/" + user } if gopath == "" { gopath = home + "/go" } // gopath may be overridden by --gopath flag on command line. flag.StringVar(&gopath, "gopath", gopath, "override default GOPATH") } func init() { if user == "" { log.Fatal("$USER not set") } if home == "" { home = "/home/" + user } if gopath == "" { gopath = home + "/go" } // gopath 可通过命令行中的 --gopath 标记覆盖掉。 flag.StringVar(&gopath, "gopath", gopath, "override default GOPATH") } 初始化 62 Methods 方法 Pointers vs. Values 指针 vs. 值 As we saw with ByteSize, methods can be defined for any named type (except a pointer or an interface); the receiver does not have to be a struct. 正如 ByteSize 那样，我们可以为任何已命名的类型（除了指针或接口）定义方法； 接收者可 不必为结构体。 In the discussion of slices above, we wrote an Append function. We can define it as a method on slices instead. To do this, we first declare a named type to which we can bind the method, and then make the receiver for the method a value of that type. 在之前讨论切片时，我们编写了一个 Append 函数。 我们也可将其定义为切片的方法。为 此，我们首先要声明一个已命名的类型来绑定该方法， 然后使该方法的接收者成为该类型的 值。 type ByteSlice []byte func (slice ByteSlice) Append(data []byte) []byte { // Body exactly the same as above } type ByteSlice []byte func (slice ByteSlice) Append(data []byte) []byte { // 主体和前面相同。 } This still requires the method to return the updated slice. We can eliminate that clumsiness by redefining the method to take a pointer to a ByteSlice as its receiver, so the method can overwrite the caller's slice. 我们仍然需要该方法返回更新后的切片。为了消除这种不便，我们可通过重新定义该方法， 将一个指向 ByteSlice 的指针作为该方法的接收者， 这样该方法就能重写调用者提供的切片 了。 方法 63 func (p *ByteSlice) Append(data []byte) { slice := *p // Body as above, without the return. *p = slice } func (p *ByteSlice) Append(data []byte) { slice := *p // 主体和前面相同，但没有 return。 *p = slice } In fact, we can do even better. If we modify our function so it looks like a standard Write method, like this, 其实我们做得更好。若我们将函数修改为与标准 Write 类似的方法，就像这样， func (p *ByteSlice) Write(data []byte) (n int, err error) { slice := *p // Again as above. *p = slice return len(data), nil } func (p *ByteSlice) Write(data []byte) (n int, err error) { slice := *p // 依旧和前面相同。 *p = slice return len(data), nil } then the type *ByteSlice satisfies the standard interface io.Writer, which is handy. For instance, we can print into one. 那么类型 *ByteSlice 就满足了标准的 io.Writer 接口，这将非常实用。 例如，我们可以通过 打印将内容写入。 var b ByteSlice fmt.Fprintf(&b, "This hour has %d days\n", 7) We pass the address of a ByteSlice because only *ByteSlice satisfies io.Writer. The rule about pointers vs. values for receivers is that value methods can be invoked on pointers and values, but pointer methods can only be invoked on pointers. 方法 64 我们将 ByteSlice 的地址传入，因为只有 *ByteSlice 才满足 io.Writer。以指针或值为接收者 的区别在于：值方法可通过指针和值调用， 而指针方法只能通过指针来调用。 This rule arises because pointer methods can modify the receiver; invoking them on a value would cause the method to receive a copy of the value, so any modifications would be discarded. The language therefore disallows this mistake. There is a handy exception, though. When the value is addressable, the language takes care of the common case of invoking a pointer method on a value by inserting the address operator automatically. In our example, the variable b is addressable, so we can call its Write method with just b.Write. The compiler will rewrite that to (&b).Write for us. 之所以会有这条规则是因为指针方法可以修改接收者；通过值调用它们会导致方法接收到该 值的副本， 因此任何修改都将被丢弃，因此该语言不允许这种错误。不过有个方便的例外： 若该值是可寻址的， 那么该语言就会自动插入取址操作符来对付一般的通过值调用的指针方 法。在我们的例子中，变量 b 是可寻址的，因此我们只需通过 b.Write 来调用它的 Write 方 法，编译器会将它重写为 (&b).Write。 By the way, the idea of using Write on a slice of bytes is central to the implementation of bytes.Buffer. 顺便一提，在字节切片上使用 Write 的想法已被 bytes.Buffer 所实现。 方法 65 Interfaces and other types 接口与其它类型 接口 Interfaces in Go provide a way to specify the behavior of an object: if something can do this, then it can be used here. We've seen a couple of simple examples already; custom printers can be implemented by a String method while Fprintf can generate output to anything with a Write method. Interfaces with only one or two methods are common in Go code, and are usually given a name derived from the method, such as io.Writer for something that implements Write. Go 中的接口为指定对象的行为提供了一种方法：如果某样东西可以完成这个， 那么它就可以 用在这里。我们已经见过许多简单的示例了；通过实现 String 方法，我们可以自定义打印函 数，而通过 Write 方法，Fprintf 则能对任何对象产生输出。在 Go 代码中， 仅包含一两种方 法的接口很常见，且其名称通常来自于实现它的方法， 如 io.Writer 就是实现了 Write 的一类 对象。 A type can implement multiple interfaces. For instance, a collection can be sorted by the routines in package sort if it implements sort.Interface, which contains Len(), Less(i, j int) bool, and Swap(i, j int), and it could also have a custom formatter. In this contrived example Sequence satisfies both. 每种类型都能实现多个接口。例如一个实现了 sort.Interface 接口的集合就可通过 sort 包中的 例程进行排序。该接口包括 Len()、Less(i, j int) bool 以及 Swap(i, j int)，另外，该集合仍然可 以有一个自定义的格式化器。 以下特意构建的例子 Sequence 就同时满足这两种情况。 接口和其他类型 66 type Sequence []int // Methods required by sort.Interface. // sort.Interface 所需的方法。 func (s Sequence) Len() int { return len(s) } func (s Sequence) Less(i, j int) bool { return s[i] < s[j] } func (s Sequence) Swap(i, j int) { s[i], s[j] = s[j], s[i] } // Method for printing - sorts the elements before printing. // 用于打印的方法 - 在打印前对元素进行排序。 func (s Sequence) String() string { sort.Sort(s) str := "[" for i, elem := range s { if i > 0 { str += " " } str += fmt.Sprint(elem) } return str + "]" } Conversions 类型转换 The String method of Sequence is recreating the work that Sprint already does for slices. We can share the effort if we convert the Sequence to a plain []int before calling Sprint. Sequence 的 String 方法重新实现了 Sprint 为切片实现的功能。若我们在调用 Sprint 之前将 Sequence 转换为纯粹的 []int，就能共享已实现的功能。 func (s Sequence) String() string { sort.Sort(s) return fmt.Sprint([]int(s)) } This method is another example of the conversion technique for calling Sprintf safely from a String method. Because the two types (Sequence and []int) are the same if we ignore the type name, it's legal to convert between them. The conversion doesn't create a new value, it 接口和其他类型 67 just temporarily acts as though the existing value has a new type. (There are other legal conversions, such as from integer to floating point, that do create a new value.) 该方法是通过类型转换技术，在 String 方法中安全调用 Sprintf 的另个一例子。若我们忽略类 型名的话，这两种类型（Sequence 和 []int）其实是相同的，因此在二者之间进行转换是合法 的。 转换过程并不会创建新值，它只是值暂让现有的时看起来有个新类型而已。 （还有些合 法转换则会创建新值，如从整数转换为浮点数等。） It's an idiom in Go programs to convert the type of an expression to access a different set of methods. As an example, we could use the existing type sort.IntSlice to reduce the entire example to this: 在 Go 程序中，为访问不同的方法集而进行类型转换的情况非常常见。 例如，我们可使用现 有的 sort.IntSlice 类型来简化整个示例： type Sequence []int // Method for printing - sorts the elements before printing func (s Sequence) String() string { sort.IntSlice(s).Sort() return fmt.Sprint([]int(s)) } type Sequence []int // // 用于打印的方法 - 在打印前对元素进行排序。 func (s Sequence) String() string { sort.IntSlice(s).Sort() return fmt.Sprint([]int(s)) } Now, instead of having Sequence implement multiple interfaces (sorting and printing), we're using the ability of a data item to be converted to multiple types (Sequence, sort.IntSlice and []int), each of which does some part of the job. That's more unusual in practice but can be effective. 现在，不必让 Sequence 实现多个接口（排序和打印）， 我们可通过将数据条目转换为多种 类型（Sequence、sort.IntSlice 和 []int）来使用相应的功能，每次转换都完成一部分工作。 这在实践中虽然有些不同寻常，但往往却很有效。 Interface conversions and type assertions 接口转换与类型断言 接口和其他类型 68 Type switches are a form of conversion: they take an interface and, for each case in the switch, in a sense convert it to the type of that case. Here's a simplified version of how the code under fmt.Printf turns a value into a string using a type switch. If it's already a string, we want the actual string value held by the interface, while if it has a String method we want the result of calling the method. 类型选择 是类型转换的一种形式：它接受一个接口，在选择 （switch）中根据其判断选择对 应的情况（case）， 并在某种意义上将其转换为该种类型。以下代码为 fmt.Printf 通过类型 选择将值转换为字符串的简化版。若它已经为字符串，我们需要该接口中实际的字符串值； 若它有 String 方法，我们则需要调用该方法所得的结果。 type Stringer interface { String() string } var value interface{} // Value provided by caller. switch str := value.(type) { case string: return str case Stringer: return str.String() } type Stringer interface { String() string } var value interface{} // 调用者提供的值。 switch str := value.(type) { case string: return str case Stringer: return str.String() } The first case finds a concrete value; the second converts the interface into another interface. It's perfectly fine to mix types this way. 第一种情况获取具体的值，第二种将该接口转换为另一个接口。这种方式对于混合类型来说 非常完美。 What if there's only one type we care about? If we know the value holds a string and we just want to extract it? A one-case type switch would do, but so would a type assertion. A type assertion takes an interface value and extracts from it a value of the specified explicit type. The syntax borrows from the clause opening a type switch, but with an explicit type rather than the type keyword: 接口和其他类型 69 若我们只关心一种类型呢？若我们知道该值拥有一个 string 而想要提取它呢？ 只需一种情况 的类型选择就行，但它需要类型断言。类型断言接受一个接口值， 并从中提取指定的明确类 型的值。其语法借鉴自类型选择开头的子句，但它需要一个明确的类型， 而非 type 关键字： value.(typeName) and the result is a new value with the static type typeName. That type must either be the concrete type held by the interface, or a second interface type that the value can be converted to. To extract the string we know is in the value, we could write: 而其结果则是拥有静态类型 typeName 的新值。该类型必须为该接口所拥有的具体类型， 或 者该值可转换成的第二种接口类型。要提取我们知道在该值中的字符串，可以这样： str := value.(string) But if it turns out that the value does not contain a string, the program will crash with a run- time error. To guard against that, use the "comma, ok" idiom to test, safely, whether the value is a string: 但若它所转换的值中不包含字符串，该程序就会以运行时错误崩溃。为避免这种情况， 需使 用 “逗号, ok” 惯用测试它能安全地判断该值是否为字符串： str, ok := value.(string) if ok { fmt.Printf("string value is: %q\n", str) } else { fmt.Printf("value is not a string\n") } str, ok := value.(string) if ok { fmt.Printf("字符串值为 %q\n", str) } else { fmt.Printf("该值非字符串\n") } If the type assertion fails, str will still exist and be of type string, but it will have the zero value, an empty string. 若类型断言失败，str 将继续存在且为字符串类型，但它将拥有零值，即空字符串。 As an illustration of the capability, here's an if-else statement that's equivalent to the type switch that opened this section. 接口和其他类型 70 作为对能量的说明，这里有个 if-else 语句，它等价于本节开头的类型选择。 if str, ok := value.(string); ok { return str } else if str, ok := value.(Stringer); ok { return str.String() } 通用性 If a type exists only to implement an interface and has no exported methods beyond that interface, there is no need to export the type itself. Exporting just the interface makes it clear that it's the behavior that matters, not the implementation, and that other implementations with different properties can mirror the behavior of the original type. It also avoids the need to repeat the documentation on every instance of a common method. 若某种现有的类型仅实现了一个接口，且除此之外并无可导出的方法，则该类型本身就无需 导出。 仅导出该接口能让我们更专注于其行为而非实现，其它属性不同的实现则能反映该原 始类型的行为。 这也能够避免为每个通用接口的实例重复编写文档。 In such cases, the constructor should return an interface value rather than the implementing type. As an example, in the hash libraries both crc32.NewIEEE and adler32.New return the interface type hash.Hash32. Substituting the CRC-32 algorithm for Adler-32 in a Go program requires only changing the constructor call; the rest of the code is unaffected by the change of algorithm. 在这种情况下，构造函数应当返回一个接口值而非实现的类型。例如在 hash 库中， crc32.NewIEEE 和 adler32.New 都返回接口类型 hash.Hash32。要在 Go 程序中用 Adler-32 算法替代 CRC-32， 只需修改构造函数调用即可，其余代码则不受算法改变的影响。 A similar approach allows the streaming cipher algorithms in the various crypto packages to be separated from the block ciphers they chain together. The Block interface in the crypto/cipher package specifies the behavior of a block cipher, which provides encryption of a single block of data. Then, by analogy with the bufio package, cipher packages that implement this interface can be used to construct streaming ciphers, represented by the Stream interface, without knowing the details of the block encryption. 同样的方式能将 crypto 包中多种联系在一起的流密码算法与块密码算法分开。 crypto/cipher 包中的 Block 接口指定了块密码算法的行为， 它为单独的数据块提供加密。接着，和 bufio 包类似，任何实现了该接口的密码包都能被用于构造以 Stream 为接口表示的流密码，而无需 知道块加密的细节。 The crypto/cipher interfaces look like this: 接口和其他类型 71 crypto/cipher 接口看其来就像这样： type Block interface { BlockSize() int Encrypt(src, dst []byte) Decrypt(src, dst []byte) } type Stream interface { XORKeyStream(dst, src []byte) } Here's the definition of the counter mode (CTR) stream, which turns a block cipher into a streaming cipher; notice that the block cipher's details are abstracted away: 这是计数器模式 CTR 流的定义，它将块加密改为流加密，注意块加密的细节已被抽象化了。 // NewCTR returns a Stream that encrypts/decrypts using the given Block in // counter mode. The length of iv must be the same as the Block's block size. func NewCTR(block Block, iv []byte) Stream // NewCTR 返回一个 Stream，其加密 / 解密使用计数器模式中给定的 Block 进行。 // iv 的长度必须与 Block 的块大小相同。 func NewCTR(block Block, iv []byte) Stream NewCTR applies not just to one specific encryption algorithm and data source but to any implementation of the Block interface and any Stream. Because they return interface values, replacing CTR encryption with other encryption modes is a localized change. The constructor calls must be edited, but because the surrounding code must treat the result only as a Stream, it won't notice the difference. NewCTR 的应用并不仅限于特定的加密算法和数据源，它适用于任何对 Block 接口和 Stream 的实现。因为它们返回接口值， 所以用其它加密模式来代替 CTR 只需做局部的更改。构造函 数的调用过程必须被修改， 但由于其周围的代码只能将它看做 Stream，因此它们不会注意到 其中的区别。 Interfaces and methods 接口和方法 Since almost anything can have methods attached, almost anything can satisfy an interface. One illustrative example is in the http package, which defines the Handler interface. Any object that implements Handler can serve HTTP requests. 接口和其他类型 72 由于几乎任何类型都能添加方法，因此几乎任何类型都能满足一个接口。一个很直观的例子 就是 http 包中定义的 Handler 接口。任何实现了 Handler 的对象都能够处理 HTTP 请求。 type Handler interface { ServeHTTP(ResponseWriter, *Request) } ResponseWriter is itself an interface that provides access to the methods needed to return the response to the client. Those methods include the standard Write method, so an http.ResponseWriter can be used wherever an io.Writer can be used. Request is a struct containing a parsed representation of the request from the client. ResponseWriter 接口提供了对方法的访问，这些方法需要响应客户端的请求。 由于这些方法 包含了标准的 Write 方法，因此 http.ResponseWriter 可用于任何 io.Writer 适用的场景。 Request 结构体包含已解析的客户端请求。 For brevity, let's ignore POSTs and assume HTTP requests are always GETs; that simplification does not affect the way the handlers are set up. Here's a trivial but complete implementation of a handler to count the number of times the page is visited. 为简单起见，我们假设所有的 HTTP 请求都是 GET 方法，而忽略 POST 方法， 这种简化不 会影响处理程序的建立方式。这里有个短小却完整的处理程序实现， 它用于记录某个页面被 访问的次数。 // Simple counter server. type Counter struct { n int } func (ctr *Counter) ServeHTTP(w http.ResponseWriter, req *http.Request) { ctr.n++ fmt.Fprintf(w, "counter = %d\n", ctr.n) } // 简单的计数器服务。 type Counter struct { n int } func (ctr *Counter) ServeHTTP(w http.ResponseWriter, req *http.Request) { ctr.n++ fmt.Fprintf(w, "counter = %d\n", ctr.n) } 接口和其他类型 73 (Keeping with our theme, note how Fprintf can print to an http.ResponseWriter.) For reference, here's how to attach such a server to a node on the URL tree. （紧跟我们的主题，注意 Fprintf 如何能输出到 http.ResponseWriter。） 作为参考，这里演 示了如何将这样一个服务器添加到 URL 树的一个节点上。 import "net/http" ... ctr := new(Counter) http.Handle("/counter", ctr) But why make Counter a struct? An integer is all that's needed. (The receiver needs to be a pointer so the increment is visible to the caller.) 但为什么 Counter 要是结构体呢？一个整数就够了。 An integer is all that's needed. （接收 者必须为指针，增量操作对于调用者才可见。） // Simpler counter server. type Counter int func (ctr *Counter) ServeHTTP(w http.ResponseWriter, req *http.Request) { *ctr++ fmt.Fprintf(w, "counter = %d\n", *ctr) } // 简单的计数器服务。 type Counter int func (ctr *Counter) ServeHTTP(w http.ResponseWriter, req *http.Request) { *ctr++ fmt.Fprintf(w, "counter = %d\n", *ctr) } What if your program has some internal state that needs to be notified that a page has been visited? Tie a channel to the web page. 当页面被访问时，怎样通知你的程序去更新一些内部状态呢？为 Web 页面绑定个信道吧。 // A channel that sends a notification on each visit. // (Probably want the channel to be buffered.) type Chan chan *http.Request func (ch Chan) ServeHTTP(w http.ResponseWriter, req *http.Request) { ch 接受之前，参见 Go 内存模 型），因此信号必须在信道的接收端获取，而非发送端。 并发 93 This design has a problem, though: Serve creates a new goroutine for every incoming request, even though only MaxOutstanding of them can run at any moment. As a result, the program can consume unlimited resources if the requests come in too fast. We can address that deficiency by changing Serve to gate the creation of the goroutines. Here's an obvious solution, but beware it has a bug we'll fix subsequently: 然而，它却有个设计问题：尽管只有 MaxOutstanding 个 Go 程能同时运行，但 Serve 还是为 每个进入的请求都创建了新的 Go 程。其结果就是，若请求来得很快， 该程序就会无限地消 耗资源。为了弥补这种不足，我们可以通过修改 Serve 来限制创建 Go 程，这是个明显的解 决方案，但要当心我们修复后出现的 Bug。 func Serve(queue chan *Request) { for req := range queue { sem QR Link Generator {{if .}} {{.}}